This application is based upon Japanese Patent Application Nos. 2000-79352 filed on Mar. 16, 2000, and 2000-382422 filed on Dec. 15, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to semiconductor physical quantity sensors for detecting a physical quantity such as acceleration or angular velocity, especially, to a capacitance-detecting type semiconductor physical quantity sensor.
2. Related Arts
A structure of a differential capacitance type semiconductor acceleration sensor of a prior art is shown in FIG. 1. A semiconductor layer of a supporting substrate 110 has a moving portion 201 and a fixed portion 301 separated from the moving portion 201 by a trench 140 formed by etching.
The moving portion 201 has suspension parts 202 fixed to the supporting substrate 110, a weight portion 210 supported by the suspension parts 202, and two sets of plural movable electrode members 240 connected to both sides of the weight portion 210.
The fixed portion 301 has plural first fixed electrode members 310 and plural second fixed electrode members 320. Each of the fixed electrode members 310, 320 is engaging with each set of the movable electrode members 240. Each of the movable electrode members 240 and the fixed electrode members 310, 320 has a detection surface and a non-detection surface opposite to each other. The detection surface of each movable electrode member 240 confronts the detection surface of each fixed electrode member 310 at one side with a detection interval 400 interposed therebetween to make a first capacitance. The non-detection surface of each movable electrode member 240 confronts the non-detection surface of each fixed electrode member 310 at the opposite side with a non-detection interval 410 interposed therebetween. The detection interval 400 and the non-detection interval 410 are defined between movable electrode members 240 and the fixed electrode members 320 at the right side of the weight portion 210 similarly to the left side. A second capacitance is formed by the detection surface of each movable electrode member 240 and the detection surface of each fixed electrode member 320.
When a physical quantity is applied to the sensor shown in FIG. 1, the weight portion 210 displaces in a displacement direction Y, whereby the detection intervals 40 change, for example, when the first capacitances increase, the second capacitances decrease, and vice versa. As a result, a differential capacitance between the first capacitance and the second capacitance changes by the applied physical quantity. The applied physical quantity is detected based on a change of the differential capacitance. Conventionally, the differential capacitance is detected as a voltage.
However, when an excessively large physical quantity is applied to the sensor, at least one of the movable electrode members 240 hits to at least one of the fixed electrode members 310 and 320, then adhesion (sticking) occurs between at least one of the movable electrode members 240 and at least one of the fixed electrode members 310 and 320 due to an electrostatic attracting force. Consequently, the sensor doesn""t work normally.
For the purpose to prevent the adhesion of the electrode members, protrusions are formed on the detection surface of at least one of the movable electrode member 240 and the fixed electrode members 310, 320. They reduce the electrostatic attracting force significantly. These protrusions are disclosed in JP-A-4-337468, 6-213924, 6-347474, 11-230985, 11-326365 and U.S. Pat. No. 5,542,295.
However, the protrusions in the prior arts don""t work sufficiently due to the following reason.
As mentioned above, a change of the detection interval 400 between the movable electrode member 240 and the fixed electrode member 310 is opposite to that of the detection interval 400 between the movable electrode member 240 and the fixed electrode member 320. As a result, the direction of the electrostatic attracting force applied to the movable electrode member 240 on the right side of the weight portion 210 is opposite to that of the electrostatic attracting force applied to the movable electrode member 240 on the left side of the weight portion 210. This structure is referred to as a non-symmetrical structure in this application.
For example, in FIG. 1, the movable electrode members 240 on the left side of the axis Y1 are attracted to an upper side of FIG. 1. To the contrary, the movable electrode members 24 on the right side of the axis Y1 are attracted to a lower side of FIG. 1. In addition, a spring restoring force caused by the suspension parts 202 works to restore the movable electrode members 240 to the initial positions against the electrostatic attracting force on each side. As a result, the weight portion 210 vibrates in a direction of the axis Y1 and rotates easily in directions indicated by curved arrows R as shown in FIG. 1.
It may occur that the movable electrode members 240 and the fixed electrode members 310 or the movable electrode members 240 and the fixed electrode members 320 adhere each other in the non-detection interval 410 when the excessively large physical quantity is applied to the sensor, although the non-detection interval 410 is larger than the detection interval 400.
This invention has been conceived in view of the background as described above and an object of the invention is to prevent an adhesion of a movable electrode to a fixed electrode due to an electrostatic attracting force even in a non-detection interval interposed therebetween when the non-detection interval is larger than a detection interval interposed between the movable electrode and an opposite fixed electrode.
According to the present invention, the sensor has a supporting substrate, a movable portion separated from a fixed portion by a trench. The movable portion has a weight portion, and movable electrodes formed on both sides of the weight portion in a displacement direction, and the movable portion is suspended above the supporting substrate. The fixed portion has first and second fixed electrodes each engaging with each of the movable electrodes for forming capacitors.
Each of the first and second fixed electrode is disposed in parallel with the movable electrodes so that the side faces thereof determine specific intervals, a detection interval and a non-detection interval larger than the detection interval, with the side faces of adjoining two of the movable electrodes. Protrusions are formed, for example, on both side faces of the first fixed electrode opposing to the side faces of the adjoining two of the movable electrodes.
According to the present invention, these protrusions prevent the adjoining two of the movable electrodes from adhering to the first fixed electrode at both of the side faces of the first fixed electrode when an excessively large physical quantity is applied to the sensor. The protrusions may be formed on the side faces of the movable electrodes facing the side faces of the first fixed electrode.
Preferably, the protrusions on the both side faces of the first fixed electrode are formed in the same size and the same shape. It may allow stable forming of the protrusions. More preferably, the protrusions on the both sides are disposed symmetrically with respect to a longitudinal direction of the first fixed electrode.